1. Field of the Invention
The present invention relates to a liquid crystal panel and a liquid crystal display using the panel. In particular, it relates to a liquid crystal panel and a liquid crystal display which is driven in a mode which aligns liquid crystal molecules having a negative dielectric anisotropy almost perpendicular to the substrates of the panel in the absence of applied voltage (vertical aligned mode, referred to as VA mode below).
2. Description of the Related Art
Liquid crystal displays are used widely as a display for various information processing devices, such as computer and TVs.
In a conventional liquid crystal display, a liquid crystal panel of a TN type, an STN type or a TFT type etc. is widely used in a mode which aligns the liquid crystal molecules having a positive dielectric anisotropy, i.e., a p type liquid crystal, parallel to the substrates opposed to each other in the absence of applied voltage.
In a liquid crystal panel of a TN mode, for example, the alignment direction (the parallel direction) of the liquid crystal molecules adjacent to one substrate is usually twisted 90 degrees to that of the liquid crystal molecules adjacent to another substrate. When a pair of polarizing plates disposed outside of the panel are arranged so that their transmission axes intersect at right angles with each other (crossed nicol), a white display is realized in a non-driving state, while a black display is realized in a driving state. On the contrary, when the transmission axes of the polarizing plates are arranged parallel to each other (parallel nicol), a black display is realized in a non-driving state while a white display is realized in a driving state.
In such a TN mode crystal display panel, it is understood that the liquid crystal molecules are aligned parallel to the substrate surface of the panel in a non-driving state, and on the contrary, they are aligned almost perpendicular to the substrate surface of the panel in a driving state. In reality, however, the liquid crystal molecules adjacent to the substrate surface of the panel keep the parallel alignment even in a driving state. The liquid crystal molecules in such a parallel alignment cause a birefringence which transmits some amount of light through the panel in a driving state. Due to this fact, a high contrast cannot be realized in the conventional TN mode liquid crystal panel.
As shown in Unexamined Japanese Patent Publication No. 8-43825, VA mode liquid crystal panels are being developed for practical application. In a VA mode liquid crystal panel, a liquid crystal having a negative dielectric anisotropy is held between a pair of substrates forming the panel in a manner that the molecules of the liquid crystal are aligned almost perpendicular to the substrates.
Since the liquid crystal molecules in this VA mode panel are aligned almost perpendicular to the substrate surface in a non-driving state, incident light passes through the liquid crystal layer without its polarization plane changing very much. Thus, when the transmission axes of a pair of polarizing plates provided to sandwich the substrates are arranged perpendicular to each other (crossed nicol), an almost perfect black display can be realized in a non-driving state. Contrary to this, a white display can be realized in a driving state due to the rotation of the polarization plane of the incident light, since the liquid crystal molecules move to align parallel to the substrates while retaining the 90 degree twisted relation between the two substrates.
As a result, a very high contrast can be obtained in such a VA mode liquid crystal panel, a level of contrast which cannot be obtained by a TN mode liquid crystal panel.
However, the VA mode liquid crystal panel has a problem that it is colored yellow when observed from an upper oblique direction. To avoid this problem, it is possible to decrease the retardation of a liquid crystal panel. However, the display contrast of the panel deteriorates as the retardation of the panel decreases, thus reducing the brightness of the panel.
The present invention has been made to overcome the above-mentioned problems of the prior art. Therefore, an objective of the present invention is to provide a liquid crystal panel and a liquid crystal display using the panel which shows a high contrast and no coloring in a white display.
The first feature of the present invention provides a liquid crystal panel which is comprised of a pair of substrates, first and second electrodes formed on inner surfaces of the respective substrates, and first and second alignment films formed on the inner surfaces of the substrates to cover the first and the second electrodes respectively. In such a panel, a liquid crystal comprised of liquid crystal molecules having a negative dielectric anisotropy is provided between the two substrates to form a liquid crystal layer that contacts the first and the second alignment films. In the first feature, the liquid crystal layer contains a certain amount of dichroic dye, which is preferably a blue dichroic dye having a strong absorption band in yellow. The amount of this dye is 0.5 to 5 wt %, preferably, 1 to 3 wt % of the amount of liquid crystal layer. Preferably, the dichroic dye is made of an anthraquinone dye. In addition, the dye may have a strong absorption coefficient in the direction parallel to the major axis of the dye molecules.
According to the first feature mentioned above, the liquid crystal molecules in the panel are aligned almost perpendicular to the substrates in the absence of applied voltage because of their negative dielectric anisotropy. The molecules of the dichroic dye are aligned parallel to the liquid crystal molecules. Therefore, an excellent black display can be obtained in the absence of applied voltage when a pair of polarizing plates are disposed in crossed nicol.
On the other hand, when a voltage to conduct a white display is applied across the first and second electrodes, the liquid crystal molecules change their alignment direction so that a considerable angle, for example, about 45 degrees, is made against the substrate surfaces. At the same time, the molecules of the blue dichroic dye change their alignment direction to keep a parallel alignment with the liquid crystal molecules. Therefore, an excellent white display is obtained when observing the panel from a direction perpendicular to the panel. In addition, when observing the panel from an upper oblique direction, a yellow coloring due to the alignment direction of the liquid crystal molecules is suppressed because of the yellow absorption characteristics of the blue dichroic dye since the molecules of this dye are aligned parallel to the alignment direction of the liquid crystal molecules.
In a prior art of the present invention, the yellow coloring was suppressed by increasing the value of retardation of the liquid crystal. In this case, however, the display contrast was deteriorated when the retardation became large. Contrary to this prior art, the present invention need not increase the retardation to suppress the yellow coloring, and a high contrast in the display can be maintained.
In the second feature of the present invention, a color compensation layer containing the dichroic dye is formed on at least one of the substrates, instead of adding the dichroic dye into the liquid crystal layer.
According to the second feature, the yellow coloring is suppressed by the color compensation layer containing the dichroic dye based on the same reason as the first feature. Therefore, a liquid crystal panel having a high contrast and substantially no yellow coloring can again be obtained.